Allow multiple Mappings windows

[Lorp]

Multiple Mappings windows will allow users to have a complete view of a variable font with more than 2 axes, with changes in all axes visualized. A button could potentially create all windows necessary for the axes in the font, so for a 20-axis font 10 Mappings windows would be created.

[davelab6]

https://developer.mozilla.org/en-US/docs/Web/API/VisualViewport could help make room by "zooming out"

[Lorp]

The main functionality, handling multiple View windows, is now implemented in a54ddec. New View windows are created with the same size as the last-created window and positioned at an offset of (20,20) from it. This has the disadvantage of obscuring previous View windows.

How about this logic for new View window layout?

• Size: take the dimensions of the last-created View window.
• Position: find space for the new View window in the document as close to the top as possible — if no rectangle large enough exists, then expand the canvas downwards to create space.

This way, we leave it to the user to decide whether very small View windows are ever created.

[Lorp]

Zooming the viewport, à la Prezi, is intriguing. However the reduction of size of UI elements (text labels, axis sliders, draggable mappings, window title bars, etc.) seems to be an overwhelming disadvantage in this situation.